Servo mechanisms



March 29, 1960 s. YANDo sERvo MECHANIsMs 2 Sheets-Sheet 1 Filed March28, 1958 A Il; Il!! Iii. Illlllllultdli rillllllillIlllll IN V EN TOR.STEPHEN YANDO ATTORNEYS.

March 29, 1960 s. YANDo sERvo MEcHANrsMs 2 Sheets-Sheet 2 Filed March28. 3.958

FIG. 5.

llll IV/ IIA 7l f' 52o. .22a

a m. a a L .w

a AKI )N1/wrox STEPHEN vANDo ATTORNEYS.

Unite States Patent SERV() MECHANISMS Stephen Yando, Huntington, N.Y.

Application March 28, 1958, Serial No. 724,683

Claims. (Cl. 121-41) This invention relates to servo mechanisms. Servomechanisms are designed to make large forces and massive equipmentsensitively and accurately responsive to light control forces. They havemany valuable uses including, for example, power brake and powersteering `strongly driven by the fluid actuator in either of two lopposite directions from a zero position, to assume a position uniquelycharacteristic of the measure and sense of the input control voltagesupplied from a control station. The output member is accordinglyarranged to control a feedback yvoltage unit in such a wayl that thefeedback voltage is always opposite in sense to the input voltage and iscau-sed to increase progressively in absolute value as the displacementof the output member from the zero position increases, until thefeedback voltage cancels out the input` voltage, causing the net voltageapplied tothe electromagnetic controll member to be zero, and therebycausing the valve to be restored to the neutral condition in which fluidcan neither enter nor leave the fluid actuator. kind involves a numberof distinct operating units of consid'erable aggregate weight and cost,each subject to its owninii'rmities, and a rather complex and elaboratecircuitry.

It is the primary object of the presentinvention to provide a servomechanism, capable oi?Y performing the services outlined above in animproved way, which mechanisni is reduced to a very simple electrical"or other etiective control means,` together with a single composite unitcomprising a control valve, a power` actuatorl and an output member.

Important advantages of the invention reside in the reduction of costdue to simplicity of organization, parts and assembly, reduction'ofYspace required, lightness of construction adapting the mechanism to beairborne, ruggedness, dependability, precision of response; andsta.-bilityV under load. f

It is an important objectA ofthe invention to providea feedbacksystemwhich acts, not to eliminate the appli cation of the yielding controlforce to the valve when the desired position of the outputl member hasbeen attained, but rather to develop automatically, as a function of theextent of displacementv of the pistonga force which will; balance thecontrol force and return, the valveV to the neutral condition when theextent of output An organization of this vof construction.

cause the feedbackr forces to operate according to any y n 2,930,360Patented Mar. 29, 1960 has of electrical circuits, which is capableofautomaticallyV opposing a progressively increasing resistance to thecon-y trolwforce, through the valve, as the displacement of the outputmember progresses, causing the control force to be balanced and thevalve to be'returned to, and stably maintained in, the neutral ornon-operating relation by said Vbalanced forces.

To these ends, and in accordance with a preferred illustrativeembodiment of the invention, itis a featurey that provision is made of aported cylinder,.a piston in the cylinder, a slide valve associated withthe cylinder andV normally occupying a neutral position relative toy theports so that uid under pressure can neither enter nor leave thecylinder, magnetic means carried Vby the valve and by the piston andarranged to cause the valve normally to occupy a neutral position, andcontrol means operable yieldingly to displace the valve, theconstruction and arrangement being such that the piston is caused to beoperated in a direction opposite tol that in which the valve has beendisplaced, and. is-ellective magnetically to repel either end of theValve which it approaches with progressively increasing force until thenet eiect of the force exertedby the piston on the valve becomesequal.vto the yieldingly applied control. force, returns the valvel toneutralposition over the resistance of said control force, and therebyestablishes a stable condition of the output member, free 'fromsubstantial hunting; effects, with the output. member in: a; deiiniteposition uniquely characteristic of the magnitude anddirection of theAcon;- trol force acting on the valve.

lt is not essential that the feedbackmeans utilize permanentmagnetsg'since electromagnets orother eld producing means may beutilized. The number and arrangement of the eld producing means may alsobe varied, asy can the valve arrangement andv various other features TheVmechanism may be contrived to desired law in relation to the extent. ofpiston, displace ment. With permanent magnets, each magnet` operatesVaccording to a square law, the repelling forcee increasing. in; inverserelation to thel square of.A the distance between thel piston carriedand valve carried polesunder consideration. This is advantageous becauseof the stability which servodevice' embodying'features of the invention,the'V partsbeing 'shown in a neutral condition;

Fig. 2 is a transverse sectional view, takeu'on 2 2 of Fig.l l, lookingin the direction of the arrows;

' Fig. 3- isl a reduced longitudinal section* similar to`V Fig. lwherein aslide valve is shown'in a position to establish liuid.connections for driving the piston upward;

Fig. 4 is a view similar to Fig. 3 wherein theslide'valve' is shown in aposition to establish connections foldriv- Ving the piston downward;

` Fig. 7vis a reduced sectionsimil'a'rltoflsigY S wherei'nltheV slidevalve is shown. in a position to establishil'uid'c'onfy nections fordriving the piston upward;.and

Fi'g: 8`is a view similar to `Fig. `7wherein the slidefvalve the line isshown in a position to establish uid connections for driving the pistondownward.

An illustrative electro-hydraulic servo device embodying features of theinvention is disclosed in Figs. 1 to 4, inclusive. The device 10comprises an elongated housing 12, shown'as of cyclindricalconfiguration. The housing 12 is fluid-tight, being closed `at its upperend by a wall 14 and at its lower end by a wall 16. The lower end wall16 is provided with an opening in which there ispmounted an inner valvesleeve 18. The valve sleeve 18 forms an integral part of the housing 12,being secured thereto at its upper end through a radial wall 20, and atits lower end through the wall 16.

. The valve sleeve 18 is provided with a plurality of inlet ports 22 and24, adjacent the walls 16 and 20, respectively. The ports let intoannular space 26 that is deiined between the sleeve 18 and the outerwall of the housing 12. Inlet port 22 provides for uid entrance into thelower portion of the longitudinally extending uid space 26, while inletport 24 serves as a Huid inlet at the upper end of the fluid space 26.

To provide for the discharge of fluid longitudinally spaced fluidexhaust or outlet ports 28 and 30 are provided in the valve sleeve 18,adjacent, respectively, to the walls and 16. The outlet port 28 permitsthe discharge of fluid from the upper portion of the longitudinal space26, while the lower outlet port 30 permits the discharge of fluid fromthe lower portion of the space 26. The annular fluid space 26 isprovided into lower and upper working chambers 32 and 34 by theinterposition of an output member in the form of a uid actuated piston36. The piston 36, which is the initial member of a power outputmechanism, is suitably secured through slide rods 38 for transmittingmotion of the piston to any external output member such as a crossbar 40and an eye member 42 which is unitary therewith. Although, in theinstant embodiment, members 38 are shown in the form of rods threaded attheir lower ends 44 to the piston 36 and at their upper ends 46 to thecrossbar 40, it will be understood that they may take any otherconvenient and practical shape.

The piston 36 is selectively moved in the working space 26 of thecylinder by the introduction of uid under pressure to either one of thechambers 32, 34 and the simultaneous discharge of fluid from the otherof said chambers. For this reason, a conduit 52 is provided as a meansfor supplying uid -under pressure from any suitable external source ofenergy such as a hydraulic pump (not shown) to the interior of thesleeve 18 through an opening 54, the opening 54 being formed in an endportion of the valve sleeve 18 which projects beyond the lower end 16 ofthe housing 12. The uid under pressure is transmitted selectively eitherto chamber 32 or to chamber 34 by a slide valve 56. The slide valve 56has a snug but free sliding t in the valve sleeve 18. A spline 56x isprovided which prevents rotation of the slide valve 56 but does notrestrain the desired sliding motion. It is provided with inlet andexhaust passages S8 and 60. The inlet passage 58 has an entrance port 62that is in constant communication with the inlet aperture 54 and thepressure fluid supply means or conduit 52. The pressure fluid istherefore in constant communication with the inlet passage 58. Thepassage S8 is provided with a uid port 64 for alignment with the inletport 22 of the valve sleeve 18, and with a tluid port 66 for alignmentwith the inlet port 24 of the sleeve 18, so that fluid under pressuremay be selectively transmitted to the chamber 32 or the chamber 34 underthe control of the slide valve 56. Normally, however, the valve occupiesa neutral position like that shown in Fig. 1, in which the passage 58communicates neither with the port 22 nor with the port 24. Thedischarge passage 60 of the slide valve 56 is provided with sidepassages or ports 68 and 70 for cooperating, respectively, with thedischarge ports 28 and 30. In the `neutral relation of Fig. l, however,the passage 60 is not in communication with either the port 28 or theport 30.

It is to be noted that unlike the inlet passage means 58, the exhaustpassage 60 extends the complete length of a comparator 88 (to be morefully described and explained presently) of which the valve 56 forms apart, opening at its upper end into a lluid receiving pressureequalization chamber 72 defined in the upper end of the housing 12between the end wall 14 and the impermeable transverse wall 20 thereof.The lower end of exhaust passage 60 opens into a uid pressureequalization chamber 74 defined in the projecting end of the sleeve 18.Stops 72x and 74x are provided at the ends of the chambers 72 and 74,respectively, for keeping the passage 60 in free communication with thechambers and for limiting the stroke of the valve 56. An outlet opening76 is provided in the projecting end of the sleeve 18 to permit thedischarge of fluid in the direction of the arrow 78 from the device 10to a receiving or recirculating chamber, not shown. Valve means, notshown, may be provided in a discharge line (not shown) for assuring thepresence of uid under predetermined limited pressure at all times in thepassage 60 and in both the chambers 72 and 74.

The position of the piston 36 and its related output member 40 isprecisely determined and maintained by the unique comparator 88 whichserves as a novel feedback means. 'Ihe comparator 88 comprises the slidevalve 56 and a plurality of eld responsive members, illustratively shownas permanent bar magnets. These include bar magnets 80 and 82 iixedlycarried and mechanically joined by the slide valve 56 beyond theopposite ends of the range of piston travel. The magnets 80 and 82 are,themselves, iield producing, but since the comparator with its magnets80 and 82 is designed to yield to magnetic and electromagnetic forces,the magnets 80 and 82 are referred to as field responsive. A further barmagnet 84, ixedly secured upon the piston 36, constitutes a furtherfield producing means and serves as one input to the comparator, theother input being an arbitrarily produced dis placing-force yieldinglyapplied to the comparator as will be more fully explained. The magnets80, 82 and 84 all have their axes disposed parallel to the axis ofcylinder 32. The magnets 80 and 82 both have their north poles disposedin one direction and their south poles disposed in the oppositedirection, but the arrangement of the magnet 84 is the reverse of thatof the magnets 80 and 82.

In the absence of other forces acting on the comparator 88, thismagnetic polar relationship causes the slide valve 56 to assume theneutral relation of Fig. l when the piston is in its median or zeroposition, as shown in Fig. 1.

Selective controlled operation and displacement of the piston 36 awayfrom the zero output position is produced by applying an extraneouslyproduced, yielding displacing force to the comparator 88 through adisplacing force applying means. The displacing force may be appliedmechanically or in any other suitable manner. As illustrated anelectromagnetic coil 86, adapted to be energized selectively by currentflow of any selected value and in either direction desired, is fixed onthe protruding end of the sleeve 18 in position to have the magnet 80received `within it to a greater or lesser degree. Displacement of thevalve 56 from neutral is etected in one direction by the flow of currentin one direction through the coil 86 and in the opposite direction bythe ow of current in the opposite direction through the coil 86, thecomparator being attracted by the action of the coil on the magnet 80 inone case and repelled by the action of the coil on the magnet 80 in theother.

Since the valve balancing forces exerted upon the comparator magnets 80and 82 in the zero position of the piston 36 are Vlight forces, even alight yielding force applied to the valve 56, mechanically or throughthe coil 86, will suice to disturb the balanced condition of thecomparator and the neutral relation of the valve tothe cylinder ports.

If the comparator 88 is yieldingly pulled downward, a condition likethat of Fig. 3 will be producedgin which inlet port 2,2 and outlet port28 are rendered active by the valve 56. The piston will be displaced inan upward direction, causing the magnet 84 to approach the com- .Kparator magnet 82 andV repel it with increasing force, and vto separatefrom the comparator magnet 80 and repel the y magnet 80 with diminishingforce.

Since the net effect of magnet 84 on the comparator system will be thevector sum of these two forces, an increasingforce tending to move thecomparator, including the valve 56, in an upward direction toward theneutral position is developed. When this developed force becomessuicient to balance the control force yieldingly applied to thecomparator 88,' as by the coil 86, the valve is restored to the normal,neutral relation of the valve to the ports of sleeve 18 illustrated inFig. l. If the pres? sure uid employed is hydraulic uid, and itpreferably is for most purposes', the piston 36 will be positivelyvlocked in a precise, predetermined position which is characteristic ofthe control force yieldingly applied to, and maintained upon, thecomparator S8 by the coil 86. With this kind of arrangement the positionof the valve is stably maintained and this, in turn causes the positionAofthe piston 36 to be stably maintained.

Should the downwardly acting control force be increased, the same valveports, 22 and 23, 'will be uncovered and a further action like the onealready described will be eiected to carry the piston further upward andlock it in a new position characteristic of the increased control force.f vA diminution of the downward control force applied to the valve 56,or reversal to an upward force applied to the valve willl cause thevalve to befmoved upward to a condition like that shown in Fig. 4 Thiscauses the inlet port 24 to be at least partially uncovered foradmitting pressure iluid to chamber 34 for driving the piston 36downward, and the discharge port 30 to be simultaneously uncovered atleast part way for permitting escape of lluid from the chamber 32. Thevalve will again be closed and the piston locked, when the net forceexerted on the comparator magnets is exactly equalV inabsolute value,but opposite in direction, to-the controlA force yieldingly applied andmaintained by the. coilSG.

In practice it frequently happens that the device 1i?. is subjected tosudden stressesv caused, for example, by, change of speed or directionof the body on which the device is carried. Since these stresses areunavoidably applied to all parts of the servo mechanism, ncludingfthehydraulic iluid andthe comparator 88, it is important that measures betaken to avoid displacement of the valve through inertia eifects. Tothis end the average density of the comparator 88 is made equalorsubstantially equal v of Figs. 1 to 4; It differs chiefly 'in thefactura the chambers at opposite Vsides of the pistondiffer substantially in their cross-sectional dimensions, that the chamber of smallercross-section is constantly and freely in communication with thepressure fluid supplying means, and

that the valve controls admission and discharge of uid r ing parts withthe subscript a added in each instance, and` Ythe description andexplanation will be substantiallyy couned to specific dilferences. Y,

Instead of the slide rods 38 and the cross bar 40 of Figs. l to 4, athick walled output sleevek 38a and a cap member, screwed thereon, aremade unitary with the piston which, in this instance, constitutes in itsentirety a bar magnet 84a of annular form. A hole 90 is provided incapmember 40a to avoid pressure andv suction elects. The sleeve has theeiect of reducing the cross,-

Y sectional area of the upper chamber 34a and the upper tween thechambers 72 and 74, and since the chambers ,I

72 and 74 are filled with hydraulic tluid atall times, the comparator 83is compelled to move in unison with, and not relative to, the iixedparts of the` servomechanism such as the housing l2 and the sleeve 1S.Thisequality of densitiesV may be achievedby employing a hydraulic iluidof comparatively high density and a valve 56 which consists of magnesiumor aluminum or an alloy of one or both of these metals, hollowed out towhatever extent maybe necessary for achieving the stated purpose.

A In Figs. 5 to 8, inclusive, another practical and advantageousillustrative form of seryo-mechanismliia einbodyinggfeatures of theinvention is disclosed. The device 10d has many features in common withthe' device 10k `ports 54a and 22a.

piston surface 50a very materially as Vcompared with the cross sectionalarea of the lower chamber 26a and theVV lower piston face 48a, desirablyto a ratio of substantially l to 2.

The conduit 52a is a'branched conduit, o'ne branch leading to port 54aand the other being` connected to communicate freely with the chamber34a at all times.`

The end wall .14a is thickened and circumferentially rabbeted, thereduced end extendingbelow the adjacent inlet of conduit 52a to preventcutting o of the port byl the piston, while maintaining a clear passagebetween conduit 52a and the upper cylinder chamber 34u.

The sleeve 18a is extended at its upper end beyondthe housing 12a andwithin the output sleeve 38a, being closed at its upper end to providethe pressure equalizing` chamber 72a. The Vsleeve 18a includes ports 22aand 30a for admitting pressure fluid to the chamber 26a and dis#`charging it therefrom, respectively, but no'ports corre-f vided. Y

The comparator valve 56a includes an intakey passage 58a having ports62a and 64a for cooperating with the A discharge passage 60a extendsthrough the comparator 88a from equalizing chamber 72a to equalizingchamber 74a, and' has a port 70afor cooperating with discharge port 30a;

The action of the comparator magnets a andk 82a; piston magnet 84a, andthe coil 86a andjtheir effects upon the valve 56a are as previouslydescribed. The hydraulic action, however, requires a word ofexplanation. When the .valve is pulled downward from the position ofFig. 5y to a position like that of Fig. 7, pressure uid is admittedspending to the ports24- and 2S ofl Figs. 1 to 4,`are proto the chamber26a. Since the-force of the uid pressurev against the lower face ofpiston S4Zz'is` greater than that` against the upper face of the piston,because of the v`difference o'f areas, the piston moves upward, forcingilu'id" in the upper chamber 34a, which is under the same pres- Sure,back into the pressure supply conduit 52a. This actionY continues untilthe valve 56a has beenrestored to the piston downward, and driving thepressure il'uid out 'i of chamber 26a until the comparator forcesbalance the control force, closing the valve and leaving the pisto'n ina new position characteristic of the magnitude and direction of thecontrol force maintained upon the valve.

While certain preferred embodiments of the invention have beenillustrated and described in detail, it is to be understood that changesmay be made therein and the invention embodied in other structures. Itis not, therefore, the intention to limit the patent to the specificconstruction illustrated, but to cover the invention broadly in whateverform its principles may be utilized.

I claim:

l. A servo mechanism comprising, in combination, means supplying fluidunder pressure, a ported, chambered housing, a fluid actuated piston inthe housing, a slide valve normally maintained in a neutral relation tothe housing ports but movable relative to the housing ports to controladmission and discharge of fluid to and .from at least o'ne chamber ofthe housing, field producing means, causing fields to be established atopposite ends of the slide valve, field producing means carried by thepiston and acting to apply a progressively increasing repelling force toeither end of the slide valve toward which the piston is moved, andcontrol means associated with the slide valve for yieldingly urging theslide valve in either o'f two opposite directions from the neutralrelation, the construction and arrangement being such that the pistonwill be fluid operated in the direction opposite to that in which thevalve is displaced to a definite position uniquely characteristic of thedirection and magnitude of the control force applied to the valve, andin which the valve is restored to the neutral relation through acounterbalancing of the control force.

2. A servo mechanism comprising, in combination, means supplying uidunder pressure, a ported cylinder, a fluid actuated piston in thecylinder, a slide valve normally maintained in a neutral relation to thecylinder ports but movable relative to the cylinder to control admissionof fluid from said supplying means to the cylinder and discharge ofiiuid from the cylinder, field responsive means carried by the slidevalve at opposite ends thereof, field producing means carried by thepiston and acting to repel either end of the slide valve toward whichthe piston is moved, and control means associated with the slide valveand operable yieldingly to urge the slide valve in either of twoopposite directions from the neutral relation, the construction andarrangement being such that the piston will be fluid operated in thedirection opposite to that in which the valve is displaced to a definiteposition uniquely characteristic of the direction and magnitude of thecontrol force applied to the slide valve.

3. A servo mechanism comprising, in combination, means supplying tiuidunder pressure, a ported cylinder, a uid actuated piston in thecylinder, a slide valve no'rmally maintained in a neutral relation tothe cylinder ports but movable relative to the cylinder to controladmission and discharge of duid to and from the cylinder, magnetic meanscarried by the slide valve at opposite ends thereof, magnetic meanscarried by the piston and acting to repel either end of the slide valveto'ward which the piston is moved, and electromagnetic means associatedwith the slide valve and energizable yieldingly to urge the slide valvein either of two opposite directions from the neutral relation, theconstruction and arrangement being such that the piston will be liuidoperated in a direction opposite o'f the valve displacement, to adelinite position uniquely characteristic of the direction and magnitudeof the control force applied to the slide valve.

4. A servo mechanism comprising, in combination, means supplyinghydraulic iiuid under pressure, a ported cylinder, a liuid actuatedpiston in the cylinder, a slide valve normally maintained in a neutralrelation to the cylinder ports but movable relative to the cylinder tocontrol admission and discharge of hydraulic fluid to and from onechamber, at least, of the cylinder, field producing means providingopposite polarities in confronting relation to the piston at oppositeends of the slide valve, iield producing means carried by the piston andacting to exert a repelling force upon either end of the slide valvetoward which the piston is moved, and control means associated with theslide valve for applying yielding force to urge the slide valve ineither of two opposite direction from the neutral relation, theconstruction and arrangement being such that the piston will behydraulically operated in a direction opposite to that in which thevalve is displaced until the valve is compelled, against the resistanceof said yielding force and while said force is still maintained, toreassume the normal relation to the cylinder ports, so that the pistonwill be caused to assume a definite position uniquely characteristic ofthe direction and magnitude of whatever control force may be applied tothe slide valve.

5. A servo mechanism comprising, in combination, means supplying uidunder pressure, a cylinder, a fixed ported sleeve extending into thecylinder and throughout the major portion of the length thereof, a fluidactuated piston in the cylinder, a slide valve having a snug sliding fitwithin the sleeve and normally maintained in a neutral relation to theports, but movable relative to the sleeve and cylinder to controladmission and discharge of hydraulic fluid to and from the cylinder,iield responsive means providing opposite poles in confronting relationto the piston at opposite ends of the slide valve, field producing meanscarried by the piston and acting to exert a progressively increasingrepelling force upon either end of the slide valve toward which thepiston is moved, and control means associated with the slide valve foryieldingly urging the slide valve in either of two opposite directionsfrom the neutral relation, the construction and arrangement being suchthat the piston will be fluid operated in a direction opposite to thatin which thc valve is displaced by the control force until the valve iscompelled, against the resistance of said yielding control force andwhile said force is still maintained, to reassume the normal relation tothe cylinder ports, so that the piston is caused to assume a definiteposition uniquely characteristic of the direction and magnitude ofwhatever control force, within the limits of the mechanism, may beapplied to the slide valve.

6. A servo mechanism as set forth in claim 5 in which the slide valvehas intake and discharge passages therein, and segregated chambers areprovided in which opposite ends of the slide valve play, the dischargepassage of the valve being extended completely through the valve fromend to end thereof in free communication with both chambers, wherebysuction effects tending to interfere with the free and intendedoperation of the valve are avoided.

7. A servo mechanism as set forth in claim 6 in which the uid ishydraulic and the hydraulic tiuid and the slide valve are of equaldensity, whereby the slide valve is rendered immune to accelerationeiects acting on the mechanism as a whole.

8. A servo mechanism as set forth in claim 5 wherein the ield responsiveand field producing means referred to consists of permanent magnetscarried, respectively, by the opposite ends of the slide valve and bythe piston, the valve carried magnets having like poles disposed in thesame direction lengthwise of the valve, and the piston carried magnethaving its poles reversely disposed.

9. A servo mechanism as set forth in claim 5 in which the sleeve and theslide valve extend beyond the cylinder at one end, and in which thecontrol means comprises a coil surrounding a protruding portion of thesleeve and adapted to be variably energized by current flow of variedstrength in either direction.

10. A servo mechanism as set forth in claim 5 in which the fluid ishydraulic and sufficient clearance ix provided between the sleeve andthe slide valve to cause a thin lm of hydraulic fluid to bemaintainedcontinuonsly between the sleeve and the valve for lubricating purposes.

11. A servo mechanism comprising, in combination, means supplyinghydraulic fluid under pressure, a cylinder, a fixed ported sleeveextending into the cylinder and throughout the major portion of thellength thereof, a fluid actuated piston dividing the cylinder into twoopposed working chambers, a slide valve having intake and dischargepassages therein, and havinga snug slidingl fit within lthe sleeve andnormally maintained in a neutral relation to the ports, but movablerelative to the sleeve and cylinder selectively to effect admission ofhydraulic uid to either chamber and simultaneous discharge thereof fromthe other, field responsive means providing opposite poles inconfronting relation to. the piston at opposite ends of the slide valve,field producing means carried by the piston and acting to exert aprogressively increasing repelling force upon either end of ther slidevalve toward which the piston is moved, and control means associatedwith the slide valve for yieldingly urging the slide valve in either oftwo opposite directions in said radial space in response to fluidpressure acting` thereon, a slide valve movable longitudinally in saidsleeve, said slide valve having passages adapted respectively to admitandrexhaust fluid under pressure, uid ports in `said sleeve communicablewith the respective valve passages to admit and exhaust Huid underpressure to and from said radial space, field responsive means on saidvalve and field producing means on the piston col operating normally toycausev said valve to close the fluid from the neutral relation, theconstructionand arrangement being such that the piston willvbehydraulically operated in a direction opposite to that-in which thevalve is displaced by the control force until the valve is compelled,againstj'the resistance of said yielding con- -trol force and while saidforce is still maintained, to reassume the normal relation to thecylinderY ports, so that the piston will be caused to assume a definiteposition uniquely characteristic of the direction and magnitude ofwhatever control force within the limits of the mechanism, may beapplied to the slide valve.`

l2. A servo Amechanism comprising, in combination, l

means supplying fluid under pressure, a cylinder, a fixed ported sleeveextending into the cylinder and throughout the major portion of thelength thereof, a uid actuated piston dividing the cylinder into opposedworking chambers, output means of substantial cross-section connected tothe piston and extending through an end of the cylinder, said outputmeans having the effect of cutting down the working area of the pistonin said chamber bya substantial amount as compared with the working areaof the piston in the other chamber, said supplying means being directlyand freely connected at all times with said one chamber, a slide valvehaving a snug sliding fit within the sleeve and normally maintained in aneutral relation to the ports, but movable relative to the sleeve andcylinder to control admission rand discharge of liuid to and from saidother chamber, field responsive means providing opposite poles inconfronting relation to the piston at opposite ends of the slide valve,field producing means carried by the piston and acting to exert aprogressively increasing repelling force upon either end of the slidevalve toward which the pistonA is moved, and control means associatedwith the slide valve for yieldingly urging the slide valve in either oftwo opposite directions from the neutral relation, the construction andarrangement being such that the piston will be fluid operated in adirection opposite to that in which the valve is displaced by thecontrol force until the valve is compelled, against the resistance ofsaid yielding control force and while said force is still maintained, toreassume the normal relation to the cylinder ports, so that the pistonwill be caused to assume a definite position uniquely characteristic ofthe direction and magnitude of whatever communication between said valveand sleeve, and yieldable control means associated with said valve foryieldingly moving the valve in either of two opposite directions fromthe normal, communication closing position, the construction andarrangement being such that the piston will be hydraulically operated inthe direction opposite to that in which the valve is displaced, to adefinite stable position uniquely characteristic ofthe direction andmagnit/nde of the control force applied to the slide valve.

l5. A hydraulic servo device comprising a source of fluid underpressure, inner and outer longitudinally extending radially spacedmembers, a piston operable longitudinally in said radial space inresponse to the huid pressure acting thereon, valve means movable insaid'inner member to communicate fluid pressure to said piston, saidpiston including field producing means, a comparator including the valveas a unitary part thereof and providing l longitudinally spaced fieldsat opposite sides of the piston and in constant cooperation with saidpiston field producing means, and yieldable control means operable toyieldingly displace saidV comparator in either of two oppositedirections from the normal communication closing position, theconstruction and arrangement being such that the piston will behydraulically operated in the direction opposite to Athat in which thevalve is displaced, to a definite stable positionV uniquelycharacteristic of the direction and magnitude of the control forceapplied to v the comparator.

16., A fluid operated servo mechanism comprising, in combination, meansproviding a supply of pressure fluid, a housing chamber, an outputpiston movable in said chamber in response to uid pressure actingthereon, a valveV movable to. control communication between said housingchamber and said fluid supply means, force field producing means on saidpiston, comparator force field responsive means on said valve, saidpiston and valve force field producing and responsive means coactingVvwith progressively increasing force as the piston approaches eitherend of the valve to move said valve to closesaid communication, andcontrol means yieldingly operable' to move the valve away from closingposition over the opposition of the piston carried force field producingmeans and to maintain the valve open until, through movement of thepiston, the valve closing effect of the piston carried force fieldproducing means upon the comparator means is sufficiently increased tooffset the yieldeither of two opposite directions in response to fluidpres- Y sure acting thereon, a housing in which the piston operates,supply means for liuid under pressure, valve means between said housingand Vsaid pressure fluid supplyfmean's operable to vary thecommunication therebetween and to control the direction of applicationof the fluid pressure on said piston, said piston having magnetic means,a comparator including the valveimeans and magnetic meanspredeterminately spaced on said valve means at opposite sides of thepiston and cooperable with :said piston magnetic means normally to movesaid valve to close said communication, and electromagnetic controlmeans operable on said valve, and variably energizable to "causeselective movement of said valve, the construction and arrangement beingsuch that the piston is caused, t0

11' assume a positionV uniquely characteristic of the energization ofthe control means.

18. A hydraulic servo device comprising, in combination, a housinghaving a longitudinally extending working space therein, a pistonmovable in said space and dividing said space into two chambers, apressure uid supply means, valve means movable in one direction to admitpressure tluid from said supply means to one of said chambers, and inthe opposite direction to discharge uid from the same chamber, acomparator including the valve means and magnetic means on said valvemeans, said piston including magnetic means coacting with saidcomparator to bias said valve means toward closed position, yieldingcontrol means operable to shift the valve in one direction to admitpressure uid to said one chamber and to shift the valve in the oppositedirection to discharge fluid from said one chamber, the arrangementbeing such that the comparator means acts progressively to balance theforce of the piston magnetic means against the control force and closethe valve in response to piston movement, and means acting on the pistoncontinuously to 4exert yielding force on the piston inthe directionopposite that in which the piston is moved by the admission of pressurefluid to said one chamber.

19. A servo mechanism comprising, in combination, an output memberpossessing field producing means, a control member also possessing fieldproducing means, the field producing means of the output member and ofthe control member being arranged to interact with one another, meansfor applying small displacing forces yieldingly in either of twoopposite directions to the control member, and an external source ofenergy for applying operating power to the output member in response todisplacement of the control member from a neutral position by saiddisplacing force applying means, the construction and arrangement beingsuch that with no displacing force applied to the control member theoutput member will be maintained in a normal or neutral position withthe control member balanced by the field producing means in a neutralposition, but that when the control member is displaced in eitherdirection from neutral by the displacing force applying means, theoutput member will be operated powerfully in the direction opposite tothat in which the control member is displaced until the output memberhas been moved to a position characteristic of the magnitude anddirection of the cornparatively small displacing force applied to thecontrol member, in which position the field producing means will havesupplied a net feed-back force to the control member through the fieldproducing means of the output member and the control member suicient tocounterbalance the applied displacing force, thereby to return thecontrol member to its neutral position and arrest the output member.

20. A servo mechanism comprising, in combination, an output memberpossessing tield producing means, a control member also possessing fieldproducing means, the field producing means of the output member and ofthe control member being arranged to interact with one another, meansfor applying small displacing forces yieldingly to the control member,and an external source of energy for applying power to the output memberin response to displacement of the control member by said displacingforce applying means, the construction and arrangement being such thatwith no displacing force applied to the control member by the displacingforce applying means the output member will be maintained in a normalposition and the control member held in a normal condition, but thatwhen the control member is displaced from its normal condition to anoperating condition by the displacing force applying means, the outputmember will be operated powerfully in the opposite direction until theoutput member has been moved to a position characteristic of themagnitude of the comparatively small displacing force applied to thecontrol member, in which position the tield producing means will havesupplied a net feed-back force to the control member suflicient to drivethe control member against the resistance of the displacing force to aneutral position.

References Cited in the tile of this patent UNITED STATES PATENTS2,372,813 Darling Apr. 3, 1945 2,638,125 Parsons May 12, 1953 2,688,953VBurns Sept. 14, 1954

